Continuous flow heaters of this type are known from DE-33 39 653 A1, for instance. The second function performed by the known continuous flow heaters apart from producing and supplying hot water to the brewing unit, namely to keep the infusion beverage controlledly warm by means of a warming plate upon completion of the brewing process, has been the subject matter of a number of protective right applications. The basic problem of the controlled keep-warm process resides in that the quantity of heat, which may amount to some 10 watts only and which is meant to compensate for the losses of heat from which the infusion beverage in the storage tank suffers, is desired to be produced by the continuous flow heater as well. However, the continuous flow heater has a relatively great heating capacity which is desirable for the brewing process and may amount to more than 1000 watts.
In order on a time average to produce a comparatively so small heating-power in a continuous flow heater of this type, the principle of the controlled keep-warm process resides in that the continuous flow heater is set to operate for a relatively short time and, subsequently, is deactivated again for a relatively long time. To this end, the continuous flow heater is switched on and off, respectively, in a known fashion by a thermostat thermally coupled to it in such a manner that, once the thermostat reaches its opening temperature, it will interrupt the voltage supply to the continuous flow heater and will reconnect it again after having cooled down to its closing temperature.
The result is that a relatively large quantity of heat is conveyed in a relatively short time to the infusion beverage from the warming plate through the bottom of the storage tank, what in turn has as a result that layers of the infusion beverage resting on the bottom of the storage tank will be heated remarkably, therefore rise upwardly in the storage tank, while cooled layers drop in the direction of the bottom of the storage tank. This causes a cycle within the storage tank due to convection so that, as a result, the entire infusion beverage can be kept warm approximately at the average temperature desired. However, this constantly performing cycle of heating and subsequent cooling has adverse effects on the flavour of the infusion beverage, in particular when the beverage is coffee.
Various proposals have been made already to minimize the adverse effects on the coffee's flavour when the coffee is kept warm in a controlled way.
Thus, it has been suggested already for keeping warm small quantities of coffee to provide apart from the thermostat available already a second thermostat with a lower opening temperature which by means of a so-termed `small-quantity switch` can be connected into the circuit for the heating element of the continuous flow heater, whereby the heating power delivered by the continuous flow heater on a time average is reduced (see DE-87 01 916 U1). However, in doing so, the heating power of the continuous flow heater is reduced also in the brewing process, from what ensues that the duration of the brewing process is prolonged without there being accomplished any appreciable favourable effects on the flavour of the infusion beverage.
Another solution intended again for keeping warm small quantities of coffee resides in starting an auxiliary heating to operate by actuating the small-quantity switch. This auxiliary heating will then heat the sole temperature controller provided when said is in its open position, the result being that the period of time is extended after which the temperature controller will return to its closing position again, so that in this case, too, the quantity of heat delivered to the warming plate on a time average is reduced (see EP-0 186 097 A1).
For the activation of the corresponding switching means to keep warm small quantities of infusion beverages, both the continuous flow heaters described in DE-87 01 916 U1 and in EP-0 186 097 A1 beside the main switch require to still actuate the small-quantity switch, without the actuation of which the infusion beverage will be subjected to relatively wide temperature variations in the keep-warm process. That is to say, in case of need the user has to actuate two switching means, what will be forgotten from time to time, on the one hand, and what is detrimental to the operating comfort, on the other hand.
DE 35 35 687 A1 discloses an apparatus which is intended not only to controlledly keep warm small quantities of an infusion beverage. In the continuous flow heater described therein, the heat connection to the warming plate is influenced in a mechanical fashion in that, by way of a bimetallic element, the distance between continuous flow heater and warming plate is adjusted such that it is larger when the continuous flow heater is very hot than it is when the continuous flow heater is less hot. As a result, the quantity of heat issued by the continuous flow heater to the warming plate will be rendered steady, that means the maxima of the issued heating power are dropped and the minima are raised.
A disadvantage common to all aforementioned measures of minimizing the adverse effects on the flavour of the infusion beverage resides in that, as a criterion for the control of the continuous flow heater, the temperature of the thermostat thermally coupled thereto is used, of which it is assumed that it rather correctly reflects the actual temperature conditions within the infusion beverage contained in the storage tank. Moreover, all of the described thermostats have a relatively great hysteresis, that means their opening temperature at which they interrupt the power supply of the electric heating element of the continuous flow heater may well be 10 or more degrees Celsius in excess of their closing temperature, at which they reconnect the electric heating element with the power supply. Likewise this feature of the used thermostats contributes considerably to the described irregularity of the quantities of heat issued per time unit by the continuous flow heater in the keep-warm process to the warming plate.